Four cement minerals are usually regarded as the major constituents of Portland cement clinker: Alite which mainly consists of tricalcium silicate (3CaO.SiO.sub.2) designated C.sub.3 S, belite which mainly consists of dicalcium silicate (2CaO.SiO.sub.2) designated C.sub.2 S, tricalcium aluminate (3CaO.Al.sub.2 O.sub.3) designated C.sub.3 A, and tetracalcium aluminate ferrite (4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3) designated C.sub.4 AF.
Throughout this specification, the quantity of the main clinker compounds have been calculated according to the Bogue formulas, corrected for the presence of SO.sub.3 in the clinker (F. M. Lea: "The Chemistry of Cement and Concrete." Edward Arnold (Publishers) Ltd., third edition London, p. 115-116). It is also possible to determine the content of C.sub.3 S directly by quantitative X-ray diffraction, although it is necessary to apply a correction allowing for the increased C.sub.3 S content due to the solid solution of Al, Fe, F, Mg, and other elements into the C.sub.3 S in order to obtain results that can be compared with the C.sub.3 S content calculated according to Bogue.
A number of minor components such as MgO, TiO.sub.2, Mn.sub.2 O.sub.3, K.sub.2 O and Na.sub.2 O are present in the clinker although they usually do not amount to more than a few percent of the cement. Two of the minor components are of particular interest: sodium oxide (Na.sub.2 O) and potassium oxide (K.sub.2 O) also known as the alkalies (although other alkali metals also exist in cement). They have been found to react with some aggregates by an expansive reaction, the so-called alkali-aggregate reaction. The products of this reaction may cause disintegration of the concrete, and necessitate costly repair or even demolition of structures.
It has been found that expansion due to alkali-aggregate reaction can be reduced or eliminated by the use of cement with low alkali content. In several countries the use of cement with high alkali content is therefore restricted by standards, recommendations and other regulations. For example, in Denmark the alkali content of cement should preferably be less than 0.8% sodium oxide equivalent to be used for ready mixed concrete and for concrete element production. For other applications, even lower alkali contents are preferred. Generally, it is desirable that the alkali content should be as low as possible.
Portland cement is typically manufactured by a process, according to which clinker is obtained by heating to partial fusion a mixture comprising limestone and a combination of various materials containing the necessary quantities of silica, alumina and iron oxide, such as sand, shale, clay or fly ash. The burning process is typically performed in a rotary kiln. The clinker is finely ground in a mill to obtain the finished Portland cement. Minor amounts of gypsum or other retarders are typically added to the mill in order to control the setting behaviour of the cement.
Cement production is a very energy-intensive process. Considerable energy is required to decompose CaCO.sub.3 to CaO and CO.sub.2, to heat the clinker to the required sintering temperature of typically 1450.degree.-1550.degree.C., and to grind the clinker to the required cement fineness. Furthermore, if wet raw materials are used, significant amounts of energy are spent evaporating the water.
Various gases, which may be undesirable from an environmental standpoint, are emitted during the production of Portland cement. The high temperatures involved results in the emission of nitrous oxides, and combustion of fuel and decomposition of limestone leads to significant release of CO.sub.2.
It is both from economical and environmental points of view desirable to reduce the energy consumption. The efforts to achieve this goal in the cement industry have mainly followed two lines:
the use of mineralisers to reduce the sintering temperature PA1 the use of extenders to decrease the amount of the clinker consumed. PA1 a) from 50% to 97% by weight (calculated on the total composition) of a Portland cement clinker, the sulfur content of which is in the range of 0.5-10% by weight expressed as SO.sub.3, and the fluorine content of which is in the range of 0.13-1.00% by weight expressed as F.sup.- ; and PA1 b) from 3% to 50% by weight (calculated on the total composition) of an extender containing a carbonate selected from calcium carbonate, magnesium carbonate and calcium magnesium carbonate, and mixtures thereof, as its main constituent and having a median particle size (d.sub.50) of below 14 .mu.m.
In cement making, the term mineraliser is used to describe substances that facilitate the sintering reactions that take place in the kiln. These substances may be added to the raw mix or may be inherent in the raw materials at certain locations. Fluxes are substances that increase the quantity of liquid phase during the sintering of the clinker, thus facilitating the formation of the clinker compounds.
Mineralisers containing fluorine such as calcium fluoride, aluminum fluoride and silicofluorides are the most studied types of mineralisers. The mineralising action may be attributed to several factors including increase of the alite formation rate, lowering of alite formation temperature and stabilization of alite crystals due to solid solution of fluoride in the alite crystals. However, it is well documented in the literature that the hydraulic activity of cements at early ages is reduced at high levels of fluoride additions.
Substances containing sulfur such as calcium sulfate and alkali metal sulfates may act as fluxes, but calcium sulfate is known to raise the temperature by which CaO and C.sub.2 S is combined to form C.sub.3 S.
Use of a combined mineraliser containing fluorine and sulfur is described in several references. These indicate that fluorine overcomes the undesirable effect of calcium sulfate mentioned above.
Reductions of the required sintering temperature of up to 200.degree. C. have been reported by use of the mentioned mineralisers. As heat is very effectively recovered in modern cement kilns, the potential energy savings are, however, relatively small; less than 5% for a 200.degree. C. reduction in burning zone temperature. However, an important benefit of the decreased burning temperature is a significant reduction in the emission of nitrous oxides.
A well-known method of producing less costly hydraulic binders is to substitute a part of the Portland cements with a so-called extender. An extender is a fine grained material with a median grain size similar to or finer than the Portland cement. Typical examples of extenders are ground limestone, natural pozzolana, pulverized fly ash (PFA), and blast furnace slag (BFS).
Although the extenders may contribute to the strength development, the dilution of the Portland cement component in the extended cements generally leads to inferior early and late strength compared with pure Portland cements. The strength of extended cements may to a certain extent be increased by milling the Portland cement component to a finer grain size which, however, results in higher milling costs and limits milling capacity. The late strength may be increased by selecting particularly reactive extenders such as certain blast furnace slags or natural pozzolana. The availability of these materials are, however, limited in many geographical regions.
Combining mineralised cement, i.e. cement produced by the use of mineralisers, with an extender could be a promising way of obtaining significant reductions of energy consumption and emission of CO.sub.2 and nitrous oxides.
GB 1 498 057 (U.S. Pat. No. 4,042,408) describes cements produced by the use of mineralisers containing fluorine and sulfur. The main advantage of the use of mineralisers described in GB 1 498 057 is not the decreased sintering temperature, but that the use of mineralisers enables the manufacture of cements with high alite contents and hence high early and late strength. The early strength of the cements described in GB 1 498 057 is further enhanced by a considerable alkali content of the clinker.
It is reported in GB 1 498 057 that some of these cements are useful for incorporating with a pulverised fly ash or a blast furnace slag extender, since the higher strength of the clinker in comparison with conventional Portland cement permits a larger addition of these extenders while achieving the same strength properties of the final mixture. The strength contribution of the extenders in question to the strength of the final mixture is, however, not improved by using the cement clinker described in GB 1 498 057 as base instead of conventional Portland cements. Thus, the dilution effect will tend to decrease the strengths of the extended cements based on the cements described in GB 1 498 057 in the same manner as extended cements based on conventional Portland cement. Consequently, combining the cement clinker described in GB 1 498 057 with fly ash or blast furnace slag is not the optimum method for producing inexpensive high strength cement compositions. Furthermore, the enhanced alkali content of the cements described in GB 1 498 057 limits their usefulness as base for extended cements because of the previously mentioned alkali-aggregate reaction.
For these reasons, it will be evident that an extended cement with production costs equal to or lower than conventional extended cements, but possessing early and late strength comparable to or higher than pure conventional Portland cement, would benefit numerous applications, in particular if the extended cement is of low alkali content. Very significant reductions of the clinker content of concrete might be achieved, both by the use of extended cement and y taking advantage of the high cement strength to reduce the cement content of the concrete without compromising concrete strength. Thus, significant reductions of cost, the amount of fuel needed, and the emission of nitrous oxide and CO.sub.2 could be attained. A further benefit of such an extended cement would be that cement production capacity could be increased without the need for investment in costly equipment because a substantial amount of clinker could be substituted by an extender while the cement maintained the strength properties of a conventional Portland cement.
From the above it can be seen that there is a substantial need for art extended cement, in particular one whose clinker portion has a low alkali content, which on the one hand may be produced at reduced costs and with reduced emissions of CO.sub.2 and nitrous oxides but on the other hand has high early and late strength and may make it possible to increase production capacity without the need for costly investments.